While profound deafness in humans and other animals usually results in a severe loss of nerve fibers and spiral ganglion cells, in most cases some residual spiral ganglion cells persist. These surviving nerve cells, which represent less than 15% of the normal complement, are the type II and type III spiral ganglion cells of Spoendlin. The present investigation will use three groups of deaf guinea pigs: (1) genetically deaf waltzing guinea pigs; (2) guinea pigs deaf from antibiotic administration; and (3) guinea pigs deaf from noise exposure. When these animals have been deaf for at least 10 months, discrete lesions will be made in their spiral ganglion on one side only. The area of the lesion and at least one sample through each half-turn of the spiral ganglion will be described. Using Fink-Heimer techniques, the central projections of residual spiral ganglion cells will be determined. The nature and location of these central projections may be important in the search for an adequate cochlear implant prosthesis, since these residual spiral ganglion cells are the only nerves that are available to be stimulated by such devices.